Influence of soil characteristics on the concentrations of Cr, Pb, and Zn in tobacco leaves from Longyan, China, and the associated predictive models

In Longyan City, Fujian Province, a hub for premium tobacco in China, our study explored the connection between soil properties and heavy metal levels in tobacco leaves, focusing on chromium (Cr), lead (Pb), and zinc (Zn). We observed that the soil’s average concentrations for Cr, Pb, and Zn were 0.165, 9.53, and 5.17 mg kg−1, respectively. In stark contrast, tobacco leaves presented elevated mean concentrations of these metals, measuring 0.936, 2.19, and 102 mg/kg for Cr, Pb, and Zn, respectively. This disparity was further highlighted by the soil’s exceedance rates for Pb and Zn at 2.93% and 0.472%, and the significantly higher enrichment coefficients in tobacco, 23.1 for Pb and 19.7 for Zn, compared to Cr, suggesting a substantial health risk. Our study identified critical soil parameters, including pH, organic matter, and levels of available potassium and phosphorus, as influential factors determining heavy metal content in tobacco. Utilizing these findings, we formulated predictive regression models for estimating Cr, Pb, and Zn concentrations in tobacco leaves. The regression models for estimating the concentrations of chromium, lead, and zinc in tobacco leaves are as detailed below: YCr = 0.889 + 0.001 * XK + 0.001 * XN − 0.005 * XOM + 0.101 * XCr (R2 = 0.852), YPb = 2.608–0.069 * XpH − 0.001 * XN + 0.008 * XZn + 0.01 * XPb (R2 = 0.738) and YZn = 101.892–0.141 * XP + 0.32 * XOM − 0.123 * XPb + 0.614 * XZn (R2 = 0.801), respectively. These offering essential tools for assessing and controlling heavy metal risks in tobacco cultivation within the region.


Introduction
The rapid population growth has driven a surge in anthropogenic activities, including industrialization, urbanization, metal extraction, and agricultural development.This has led to an accumulation of heavy metals in the environment, posing threats to farmland ecosystems and elevating concerns about global heavy metal pollution [1,2].Unlike overt pollution in water and air, soil heavy metal contamination often remains undetected until it affects human health through crop consumption.Zheng et al (2020) confirmed the potential health risks from consuming crops with heavy metals [3].Once in the soil, heavy metals resist degradation and accumulate, posing long-term ecological threats.Beyond certain levels, they can harm the soil-plant system, degrade soil, reduce crop yield and quality, and contaminate water sources [4][5][6].Heavy metal stress can lead to micronutrient deficiencies in plants and reduce enzyme availability, especially in cash crops.High concentrations of heavy metals can hinder the uptake and movement of essential minerals, affecting plant growth and nutrient absorption.This not only impacts crop growth but also poses health risks upon consumption [7][8][9].However, the impact of tobacco consumption on metal accumulation in humans has been overlooked.Inhaling heavy metals from tobacco leaves through cigarette smoke can harm human health [10].
Metals like Chromium (Cr), Lead (Pb), and Zinc (Zn) exhibit toxicity when their concentrations exceed established safety thresholds.For instance, the World Health Organization (WHO) sets the maximum permissible limits in drinking water as 50 μg/l for Cr, 10 μg/l for Pb, and 3 mg/l for Zn.Exceeding these levels can lead to various health risks, including nutrient depletion, organ damage, and cancer [11,12].Awareness and regulation of these metals are vital for public health [13][14][15], emphasizing the need for research on heavy metal accumulation in tobacco.
Longyan City, renowned for its premium tobacco production, has been extensively studied.Previous research has focused on soil nutrients, pH fluctuations, tobacco leaf aromatics, environmental factors, and the influence of soil microorganisms on tobacco quality.For instance, Liu et al (2016) reported on the variability of soil nutrients in Longyan [16], while Qian et al (2019) explored soil pH dynamics [17].Chen et al (2018) identified key aroma components in Longyan tobacco leaves [18], and Lai et al (2015) examined the benefits of intercropping garlic for soil health and tobacco quality [19].Despite these comprehensive studies, there remains a notable gap in systematic analysis of the impact of soil attributes, particularly heavy metals, on the specific heavy metal content within tobacco leaves in real-world settings.This gap in research hinders effective heavy metal management in tobacco cultivation and sustainable tobacco production practices.Our study, conducted in Longyan City, aims to bridge this gap by establishing a clear relationship between the concentrations of Chromium (Cr), Lead (Pb), and Zinc (Zn) in tobacco leaves and various soil factors, including pH, Alkalihydrolyzable Nitrogen, Available Phosphorus, Available Potassium, and Organic Matter.As the field of predicting heavy metal accumulation in plants grows [14,15], this paper seeks to develop quantitative models, providing a foundational basis for assessing metal contamination in tobacco production in this region.

Background of the study area
Longyan City, situated in north-central Fujian Province, China, lies between latitudes 24°23′ to 26°02′ N and longitudes 115°51′ to 117°45′ E (refer to figure 1).Covering an area of 19,050 km 2 , it spans approximately 192 km from east to west and 182 km from north to south, accounting for 15.7% of the province's total area.The topography comprises 14,964 km 2 of mountains, 3,101 km 2 of hills, and 985 km 2 of plains, with elevations higher in the eastern and western regions and lower in the northern and southern zones.
The climate of Longyan's tobacco-growing areas exhibits a temperate subtropical marine monsoon influence, ensuring moderate temperatures year-round.The average annual temperature varies between 18.7 °C and 21.0 °C.Concurrently, annual precipitation ranges from 1031 mm to 1369 mm, with sunshine durations between 1804 and 2060 h.Such climatic conditions favor the cultivation of subtropical crops and trees [20].The predominant agricultural system in this region is a rice-tobacco rotation, optimized for tobacco growth [21].Established in the late 1970s to early 1980s, the tobacco industry in Longyan City has become one of its primary economic pillars, significantly impacting agricultural output.According to the ecological zoning standards for flue-cured tobacco set by the Tobacco Research Institute of the Ministry of Light Industry, Longyan is recognized as one of China's top three ecologically suitable regions for flue-cured tobacco production [22].

Sample collection
Flue-cured tobacco sampling sites within Longyan City included Shanghang County, Changting County, Yongding District, Liancheng County, Wuping County, and Zhangping City, as depicted in figure 1.A total of 852 soil samples were collected from a depth of 20 cm using the plum spot sampling technique.The number of sampling points in each area was determined based on the size of the tobacco cultivation area.Specifically, Shanghang County had 134 sampling points, Changting County had 138, Yongding District had 128, Liancheng County had 155, Wuping County had 64, and Zhangping City had 233.This method involves collecting five samples from a unit area, positioned according to the cardinal directions of east, west, and east.Detailed sampling procedures and specific cases are documented in [23].
Soil samples were air-dried, purified from impurities, ground, sieved with 2 mm mesh, mixed, and stored in plastic containers.Concurrently, tobacco leaf samples, specifically from the middle leaves of the plants, were collected from the same sites.These leaves were cleaned with ionized water, subjected to heat treatment between 100 °C-150 °C for 5-10 min, dried at 70 °C-80 °C until a consistent weight was reached, and then ground to a particle size of 0.149 mm.The processed samples were labeled and stored in sealed plastic bags for further analysis.

Analysis method
Soil samples were meticulously prepared and analyzed through a detailed, multi-step process.Initially, the samples underwent a digestion method involving Hydrochloric acid (HCl), Nitric acid (HNO 3 ), and Perchloric acid (HClO 4 ) to effectively break down the soil matrix and release heavy metals.This process, described in detail below, includes stages of digestion, filtration, and specific techniques for determining Cr, Pb, and Zn concentrations.Digestion: We treated each soil sample with a mixture of HCl, HNO 3 , and HClO 4 , heating the mixture to facilitate the breakdown of organic matter and release metal ions into solution.Filtration: After digestion, the liquid was filtered to remove undissolved particles, ensuring that only the clear solution containing the dissolved metals was used for further analysis.Determination of Pb: A subset of the filtered samples underwent an extraction process using Diethylenetriamine Pentaacetic Acid (DTPA) to bind with Pb ions, enhancing their detectability.The Pb levels were then quantified using Graphite Furnace Atomic Absorption Spectrometry.Assessment of Cr and Zn: The concentrations of Chromium (Cr) and Zinc (Zn) were determined using a 0.1 mol l −1 HCl extraction, followed by analysis with Atomic Absorption Spectrometry (AAS) using the PinAAcle 900 F model from PerkinElmer Singapore Pte Ltd, Singapore.
For the quantification of Chromium (Cr), Lead (Pb), and Zinc (Zn) in tobacco leaves, we utilized the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), specifically the PerkinElmer-Optima 8300 model.The detailed analytical procedure is as follows.Sample Preparation: Each tobacco leaf sample was weighed (0.2000 g), and then 5 ml of nitric acid along with 1 ml of perchloric acid was added.The sample was left to rest overnight with a small funnel placed at the bottle's mouth to facilitate gradual digestion.Digestion Process: After the initial resting period, ultra-pure water was added to the sample, which was then further dissolved until reduced to about 1 ml.This step ensured complete digestion of the leaf material, releasing the metals into the solution.Sample Standardization and Dilution: The digested sample was then transferred to a volumetric flask for volume standardization.This involved diluting the sample to a specific volume, ensuring a consistent concentration for analysis.ICP-OES Analysis: With the sample prepared and standardized, Cr, Pb, and Zn concentrations were measured using the ICP-OES instrument.This technique provides accurate quantification of metal concentrations in the sample.
The evaluation of the soil's physical and chemical properties was conducted using specific methodologies, detailed as follows.Soil pH: The pH of the soil was determined potentiometrically using the P901 pH meter from Shanghai Yuke Instrument and Equipment Co., Ltd.This was done with a 1:5 soil-to-water ratio (by weight and volume), ensuring accurate measurement of the soil's acidity or alkalinity.Organic Matter Content: The content of organic matter in the soil was analyzed using the Elementer Vario MAX CN element analyzer, a reliable method for determining carbon and nitrogen content in soil samples.Alkali-hydrolyzed Nitrogen: For nitrogen availability, we employed the alkali-hydrolyzed diffusion method.This technique involves treating the soil with an alkali to release ammonium, which is then measured to estimate nitrogen content.Available Phosphorus: The quantification of available phosphorus was achieved through spectrophotometric analysis post-sodium bicarbonate extraction.This method effectively extracts the phosphorus for accurate measurement.Available Potassium: To determine the available potassium, soil samples were extracted with 1 mol l −1 ammonium acetate and then analyzed using a flame photometer, which quantifies potassium content based on its emission spectrum.
The methodologies employed in our study, meticulously adapted and expanded from [24][25][26][27][28][29][30], were instrumental in providing a comprehensive analysis of the soil samples.These methods enabled us to accurately assess the content of Pb, Cr, and Zn, thereby offering insights into the heavy metal pollution levels of the area.Simultaneously, they facilitated a thorough evaluation of relevant soil properties, ensuring both the accuracy and the relevance of our findings.

Statistical analysis
The Pearson correlation and multiple linear regression analyses were conducted utilizing Statistical Product and Service Solutions (SPSS) (version 20.0) software.The examination of Cr, Pb, and Zncontent characteristics within the soil was performed through the application of ArcGIS 10.8 and Office 2019 software suites.

Physical and chemical properties for soil
Studies indicate that optimal plant root growth and nutrient absorption require specific soil conditions, including appropriate pH levels, available phosphorus, organic matter, alkaline nitrogen, and available potassium [31,32].Analysis of soil from the Longyan tobacco region reveals attributes consistent with the acidic terrains of southern China [33].The soil is rich in organic matter, and essential nutrients such as available potassium, Alkali-hydrolyzable nitrogen, and available phosphorus are within suitable ranges (table 1).The soil pH ranges from 3.39 to 8.57, with an average of 5.87 and a low coefficient of variation (7.66%).Notably, 96% of the soil samples have a pH between 5.50 and 6.50, indicating a predominantly weakly acidic nature, favorable for tobacco cultivation and characteristic of southern Chinese soils.The content of organic matter, Alkalihydrolyzable nitrogen, available phosphorus, and available potassium in the region varies but averages 27.7 g/k g, 10 1 mg kg −1 , 40.1 mg kg −1 , and 113 mg kg −1 , respectively.Excluding pH, which has a 7.66% variation coefficient, the variation coefficients for the other soil components are relatively high, ranging from 42.4% to 55.2%.

Heavy metals in soil
A comprehensive assessment of soil from the study area was performed to determine the concentrations of heavy metals: Cr, Pb, and Zn, as detailed in table 2. The analysis revealed an average Cr concentration of 0.165 mg kg −1 in soil, with a range extending from 0.001 mg kg −1 to 1.95 mg/kg.This variation can be attributed to differences in soil composition, historical agricultural practices, and potential industrial influences in different sampling locations.The coefficient of variation, calculated at 112%, reflects these diverse environmental factors affecting Cr distribution in the soil.Despite this variation, all samples exhibited Cr concentrations below the threshold of 2.50 mg kg −1 , which is the second-level index for agricultural soil heavy metal contamination in Fujian Province.This threshold is based on regional soil quality standards and is used to assess potential risks to human health and agriculture.The consistent findings of Cr levels below this index across all samples strongly suggest that there is no significant Cr contamination in the soils of the region studied.Such results are indicative of a relatively stable and safe level of Cr in these soils, posing minimal risk to agricultural practices and local ecosystems.In contrast, Pb concentrations were higher, averaging 9.53 mg kg −1 and varying from 0.010 mg kg −1 to 73.2 mg/kg.The coefficient of variation for Pb was 77.3%.While most samples had Pb levels below the secondary index for heavy metal contamination in Fujian Province (28.0 mg kg −1 ), 2.93% exceeded this threshold, indicating a relatively high amount of Pb contamination.
Zn concentrations were moderate, averaging 5.17 mg kg −1 , with values ranging from 0.001 mg kg −1 to 56.7 mg/kg.Although the majority of samples had Zn levels below the secondary index for Fujian Province (28.0 mg kg −1 ), a small fraction (0.0472%) exceeded this limit.
The above results show that the heavy metal concentrations in the study area's soil are generally within acceptable ranges, posing minimal risk to tobacco cultivation.

Heavy metals in tobacco leaves
Table 2 presents the concentrations of Cr, Pb, and Zn in the tobacco leaves from the study area.The Cr concentration in the leaves was the lowest, averaging 0.936 mg kg −1 and ranging from 0.010 to 2.91 mg kg −1 , with the highest coefficient of variation at 43.9%.The Pb concentration in the leaves was moderate, with values between 0.231 and 8.88 mg kg −1 and an average of 2.19 mg kg −1 .Its coefficient of variation was 54.3%.The Zn concentration was the highest, varying from 6.25 to 265 mg kg −1 and averaging 102 mg kg −1 .However, its coefficient of variation was the lowest at 48.6%.
The enrichment coefficient for tobacco leaves is the ratio of heavy metal concentrations in the leaves to those in the soil.In the Longyan tobacco region, the average enrichment coefficient for Cr was 5.67.For Pb and Zn, the coefficients were 23.1 and 19.7, respectively.These data indicate that tobacco leaves have a strong affinity for accumulating Pb and Zn from the soil.In contrast, our analysis reveals a lesser tendency for tobacco leaves to absorb Cr.Consistent with this, even though tobacco is considered a weak accumulator of Cr, its ability to absorb heavy metals from the soil cannot be ignored.

Correlation
Table 3 delineates the associations between heavy metal concentrations in tobacco leaves and their bioavailable levels in soil.The results indicate a robust positive correlation between Cr concentrations in tobacco leaves and soil, evidenced by a correlation coefficient of 0.619 (P < 0.001).Furthermore, Cr concentration in tobacco leaves correlates positively with Alkali-hydrolyzable nitrogen (0.418) and potassium (0.135), but negatively with organic matter (−0.843) in soil.
The Pb concentration in tobacco leaves also exhibits a strong positive correlation with its level in soil, as indicated by a correlation coefficient of 0.604 (P < 0.001).This concentration, however, is negatively associated with soil pH (−0.127) and N (−0.131), but positively with soil Zn (0.199).
For Zn, its concentration in tobacco leaves correlates positively with OM, Pb, and Zn in soil.In contrast, a notable inverse relationship is observed between the Zn concentration in tobacco leaves and phosphorus in soil (−0.371).

Prediction models
The empirical models, developed using stepwise linear regression as illustrated in figure 2, indicate that soil's physical and chemical properties, including organic matter content, significantly influence the concentrations of Cr, Pb, and Zn in tobacco leaves.This finding is consistent with the research conducted by Yang et al (2023), which demonstrated a correlation between soil organic matter and the concentrations of heavy metals like Cr in soil, and Tong et al (2022), who reported that organic cultivation practices led to increased levels of heavy metals such as Zn in soils.These studies underscore the critical role that soil composition, particularly organic matter, plays in the absorption of specific heavy metals by plants.The analysis highlights that soil alkali-hydrolyzable nitrogen, available Potassium, organic matter, and soil Cr content are the primary determinants for Cr concentration in tobacco leaves.In contrast, the Pb concentration in tobacco leaves is chiefly influenced by soil pH, alkali-hydrolyzable nitrogen, Pb, and Zn levels.For Zn concentration in tobacco leaves, the primary influencing factors are available phosphorus, Organic Matter, Pb, and Zn content in soil.
According to the derived regression equation, the predictive value fitting for the Zn content in tobacco leaves exhibits the highest accuracy, while the fitting accuracy for the Cr and Pb content is comparatively suboptimal, as detailed in table 4.

Correlation of heavy metal uptake in tobacco with soil properties
The assimilation of heavy metals into tobacco is primarily governed by two distinct mechanisms: atmospheric deposition onto the leaf surface and absorption from the soil matrix through the plant roots.Passioura (2002) postulated that the physicochemical attributes of the soil predominantly dictate the assimilation of heavy metals by plants [34].It is imperative to note that the concentrations of Cr, Pb, and Zn within tobacco leaves exhibit pronounced variations across different tobacco cultivation regions.Such variations are consistent with earlier studies that underscore the soil as the predominant reservoir of these metals in tobacco.While there are variations in the concentrations of Cr, Pb, and Zn in tobacco leaves from different production regions in China, these differences should be interpreted with caution [35,36].According to Zhang et al (2012), who analyzed heavy metal content in tobacco leaves across ten provinces, Fujian's tobacco leaves showed higher Pb concentrations compared to many, but not all, regions, with Hunan being a notable exception [37].Regarding Cr, the concentration in tobacco leaves from southeastern regions, including Fujian, was generally lower than in other areas.For Zn, the concentration was found to be relatively consistent across various regions.In the context of our study, focusing on the southern region, it was discerned that the concentrations of Cr and Zn in tobacco leaves were notably elevated compared to other regions, whereas the Pb concentration was moderate.The average concentrations were determined to be 0.936 mg/kg for Cr, 2.19 mg/kg for Pb, and 102 mg/kg for Zn in tobacco leaves.
Within the intricate soil-plant nexus, the sequestration of Cr, Pb, and Zn in plants is modulated by a myriad of determinants, encompassing soil pH, the content of organic matter, and the concentration of metals in the soil [38,39].Among these, soil pH stands out as a cardinal factor orchestrating the transfer of heavy metals to plants [40,41].This parameter has the capacity to regulate the concentrations of Cr, Pb, and Zn in tobacco leaves by dictating the solubility and hence the availability of these metals in the soil solution.Acidic soils, characterized by lower pH values, typically manifest heightened concentrations of heavy metal ions, thus amplifying their  bioavailability for assimilation by tobacco roots.Conversely, alkaline soils, characterized by elevated pH values, tend to have attenuated concentrations of these ions, leading to a reduced propensity for their accumulation in tobacco leaves.The influence of soil pH transcends this direct effect; it also modulates other soil attributes, notably the content of organic matter, which in turn impacts the bioavailability and sequestration of heavy metals in tobacco plants.Existing literature underscores that soil pH exerts a pronounced influence on the adsorptive capacity of soils for Cr and Pb [42].Within a pH spectrum of 4.00-7.70,the adsorptive affinity for Cr and Pb amplifies threefold for each incremental unit of pH, thereby curtailing the mobility of these metals towards the plant root zone.Golia et al (2019) accentuated the paramount significance of soil pH in dictating heavy metal dynamics within the plant-soil interface [43].For instance, in experimental setups encompassing carrot-soil, lettuce-soil, and spinach-soil systems, pH emerged as the preeminent soil attribute influencing Pb uptake, succeeded by the influence of soil organic matter [44].In the context of our study, wherein a substantial 96% of the soil samples exhibited pH values ranging from 5.50 to 6.50, with a mean pH of 5.87, a salient inverse correlation was observed between soil pH and Pb content in tobacco leaves.In contrast, Cr and Zn did not display any marked correlation with pH.This observation intimates that soils with a moderate acidic nature potentiate the absorption and subsequent accumulation of Pb in tobacco plants.This inference aligns with the findings of Shao et al (2019), who delved into the ramifications of biochar application on the growth dynamics of flue-cured tobacco, properties of the rhizosphere soil, and the metal content in leaves [45].
The content of organic matter, in conjunction with soil nutrient constituents such as nitrogen, phosphorus, and potassium, plays a pivotal role in modulating the sequestration of heavy metals in tobacco leaves [46,47].Typically, an augmentation in the soil's organic matter content leads to alterations in the bioavailability and mobility of heavy metals, primarily due to processes of complexation and adsorption [48][49][50].This phenomenon can be attributed to the propensity of soil organic matter to form stable chelates with heavy metals.Such chelation can transmute the speciation of exchangeable and water-soluble heavy metals in the soil matrix, thereby dictating their bioavailability and mobility.Furthermore, the inherent high surface area and porosity of organic matter amplify the soil's adsorptive capacity for heavy metals.Consequently, as the organic matter content escalates, the soil's affinity to adsorb metals like Cr, Pb, and Zn intensifies, leading to a concomitant reduction in their concentrations in the soil solution.Our empirical data elucidate a dichotomous relationship between soil organic matter content and the concentrations of Cr and Zn in tobacco leaves, with a negative correlation observed for Cr and a positive one for Zn.Intriguingly, Pb concentrations in tobacco leaves did not exhibit any discernible correlation with organic matter, hinting at the influence of distinct mechanistic pathways.In the context of Cr, the chelates formed between Cr and organic matter might impede its bioavailability, thereby attenuating its uptake by tobacco leaves.This is premised on the notion that such chelates might pose challenges for plant absorption.Certain edaphic microorganisms possess the capability to reduce the more bioavailable form of Cr, namely Cr(VI), to its less bioavailable counterpart, Cr(III).The presence of organic matter potentially acts as a catalyst in this reduction process, consequently diminishing the bioavailability of Cr.Moreover, organic matter accentuates the soil's adsorptive capacity for Cr, leading to a decrease in its concentration in the soil solution and, by extension, its uptake by tobacco leaves.Contrastingly, for Zn in tobacco leaves, specific components of soil organic matter, notably humic and fulvic acids, might chelate with Zn to form complexes that are more amenable to plant absorption, given their water-soluble nature.This results in an elevation of the concentration and bioavailability of Zn in soil solutions.The quintessential component of organic matter, humus, interacts with heavy metals via its constituents, humic acid and humin.These interactions, especially with humic acid and humin, are typically less soluble compared to those involving fulvic acid.Such metal-organic complexes can bolster the bioavailability of heavy metals by increasing the concentration of soluble heavy metal ions in the soil solution, facilitated by mineral surface dissolution processes.As the solubility of these complex surges, metals like Cr, Pb and Zn in the soil are more efficiently translocated to plants.Our empirical findings underscore a robust correlation between the Cr content in tobacco leaves from the studied region and soil organic matter, mirroring trends observed in extant literature.For instance, studies by Regassa and Chandravanshi (2016) on tobacco leaves from distinct Ethiopian regions (Billate and Shewa Robit) revealed a positive correlation between average Cr (1.45) and Zn (44) content and soil organic matter, with Pb being an exception [51].Similarly, Liu et al (2017) discerned a significant association between Cr, Zn, and organic matter in tobacco cultivation zones in Southwest China [52].
The concentrations of nitrogen, phosphorus, and potassium in the soil exert a profound influence on the levels of Cr, Pb, and Zn in tobacco leaves by modulating plant growth dynamics and nutrient assimilation processes.An enrichment in these cardinal nutrients can invigorate plant growth, thereby amplifying the absorption of heavy metals and culminating in augmented accumulation within tobacco leaves.In stark contrast, a paucity of these nutrients can stymie plant growth, attenuating the absorption and subsequent accumulation of heavy metals in the foliage.Furthermore, a disequilibrium in the concentrations of nitrogen, phosphorus, and potassium can perturb the homeostasis of other soil constituents, which in turn can impinge upon the bioavailability and assimilation of heavy metals.A seminal study by Wu et al (2020) employed pot experiments to delineate the nexus between heavy metal concentrations in tobacco leaves and intrinsic soil properties in Changsha, China [53].Their findings illuminated that the concentrations of Pb and Zn in tobacco leaves from this locale were significantly modulated by the soil's nitrogen, phosphorus, and potassium content, evincing a tangible correlation.This underscores the pivotal role of these nutrients in the soil matrix in shaping the heavy metal profile of tobacco leaves.Our empirical data corroborate this, revealing that the levels of nitrogen, phosphorus, and potassium in the soil have a palpable impact on the concentrations of Cr, Pb, and Zn in tobacco leaves.This suggests that maintaining optimal levels of these nutrients is imperative for the judicious cultivation of tobacco leaves in the region, particularly in the context of heavy metals such as Cr and Zn.
Soil metal concentration emerges as a salient determinant governing the sequestration of heavy metals in plants.Typically, an elevation in soil metal concentrations propels an increase in the uptake of heavy metals by plants.However, this relationship can manifest nuances contingent upon other soil attributes, notably pH and organic matter content.In the purview of our study, a positive correlation was discerned between the concentrations of Cr, Pb and Zn in both soil and tobacco, providing insights into this intricate relationship.The academic landscape is replete with research endeavors probing this domain.For instance, a comprehensive study by Duan et al (2021) entailed the collection and analysis of 107 soil and tobacco leaf samples from South China, aiming to expound upon the quantitative interplay between soil attributes and heavy metal concentrations in tobacco leaves [54].Their findings not only evinced a positive correlation between the two entities but also furnished empirical underpinnings for regression models predicated on soil parameters to prognosticate the levels of Cr, Pb and Zn in tobacco across disparate regions.

Risk and prevention of heavy metals
The rigorous assessment and strategic mitigation of heavy metal threats within agroecosystems have perennially occupied a central position in environmental and agricultural research [55,56].This heightened interest emanates from the intricate interplay of variables within agricultural terrains, encompassing fluctuations in soil heavy metal concentrations, physicochemical properties, climatic variables, topographical nuances, soil taxonomies, and agronomic practices [57,58].Among these multifarious determinants, soil emerges as a cardinal factor influencing economically significant crops, such as tobacco, positioning it as a linchpin for addressing and mitigating the challenges posed by heavy metals.
Delving into the spectrum of heavy metals, empirical studies have delineated a specific bioaccumulation hierarchy in tobacco leaves, which can be articulated as: Zn > Cd > Hg > Pb > Cr > As [59].This hierarchy accentuates a pronounced propensity for the sequestration of Pb and Zn within tobacco foliage.In mature tobacco specimens, the repositories for Pb and Zn are predominantly the roots and leaves, with the stems harboring relatively diminished concentrations [60].Given that tobacco products predominantly harness the leaves, the meticulous regulation of Pb and Zn concentrations becomes an imperative in the sustainable cultivation of tobacco.Pb, devoid of any recognized biological utility in plants, is a potent toxicant and ranks among the most deleterious contaminants afflicting ecosystems.Agricultural terrains tainted with lead can precipitate its accumulation in staple crops, encompassing rice, corn, wheat, and notably, tobacco, thereby establishing a potential conduit for human lead exposure.Conversely, Zn, an indispensable micronutrient, confers health benefits when ingested in judicious quantities.Yet, an overabundance of Zn intake can precipitate health adversities, ranging from gastrointestinal perturbations to immunological suppression.It is noteworthy, however, that the Zn concentrations in tobacco seldom reach thresholds that would elicit these health concerns, with the health ramifications of smoking overshadowing such risks.Consequently, the imperative to address the pernicious effects of Pb assumes paramount importance, necessitating a comprehensive evaluation of potential health hazards spanning the entire tobacco-food supply continuum.
The intricate dynamics of agroecosystems, particularly concerning heavy metal contamination, have garnered significant attention in contemporary scientific discourse.Our study, conducted in the Longyan area, unveiled pronounced levels of Pb and Zn in tobacco leaves, with averages of 2.19 mg/kg and 102 mg kg −1 , respectively.These findings, juxtaposed with a soil Pb excess rate of 23.1% and a Zn excess rate of 19.7%, provide a coherent explanation for the observed heavy metal concentrations in the tobacco leaves.In the context of tobacco cultivation, the repercussions of elevated Pb and Zn concentrations in the soil are multifaceted.Firstly, these heavy metals can stymie the growth of tobacco, with the roots being particularly susceptible, as demonstrated in studies such as Zoffoli et al (2012), who explored the inputs of heavy metals in tobacco fields and their impact on plant growth [61].Secondly, physiological processes, such as photosynthesis and mineral absorption, can be adversely affected, leading to diminished chlorophyll content and a disruption in mineral nutrient equilibrium.Lastly, the quality and safety of the tobacco are compromised.The accumulation of Pb and Zn in tobacco leaves not only affects the quality of the tobacco but also heightens the risk of smokers ingesting these toxic metals, thereby posing significant health threats.Historical endeavors aimed at mitigating the excessive presence of Pb and Zn in tobacco leaves have predominantly revolved around soil remediation.A plethora of studies have underscored the efficacy of both organic and inorganic amendments in curtailing the bioavailability and accumulation of these heavy metals in tobacco plants.For instance, Wang et al (2023) demonstrated that silicon fertilizers could significantly reduce Pb concentrations in both soil and plant tissues [62].Similarly, Rong et al (2020) and Xiao et al (2019) highlighted the potential of humic acid and a novel stabilizer, 2,4,6-trimercaptotriazine, trisodium salt, nonahydrate (TMT), respectively, in reducing the bioavailability of Pb and Zn in soils [63,64].Angelova et al (2019) investigated the effects of various treatments on tobacco plants, including a control (without organic ameliorants) and both compost and vermicompost amendments (applied at 2.5% and 5%, recalculated based on dry soil weight) [65].Other studies, such as those by Shao et al (2019) and Cheng et al (2018), have emphasized the benefits of biochar in promoting tobacco growth and reducing heavy metal content in tobacco leaves [66,67].These synergistic effects plausibly contributed to the diminution in the mobility and bioavailability of both Pb and Zn in calcareous soil.Wu et al (2012) conducted a field study to investigate the influence of potassium fertilizer combined with a nutrition regulator (NR) on flue-cured tobacco yield and its leaf content of potassium and heavy metals (Pb and Zn) [68].Their findings revealed that this combined treatment notably elevated the yield and potassium levels in the tobacco leaves while substantially reducing the concentrations of heavy metals (Pb and Zn).
In addition to soil remediation methods for reducing Pb and Zn levels in tobacco plants, a variety of strategies can be implemented, including Plant Selection and Breeding, Water Quality Management, Phytoremediation and Crop Rotation, as well as Biological Interventions.For instance, a study by Shafiq et al (2020) delved into the uptake of heavy metals, growth, and protein expression in two tobacco cultivars subjected to heavy metals (Cd, Cr, and Pb) and EDTA [69].This research revealed significant impacts on growth parameters and heavy metal accumulation due to the application of heavy metals and EDTA, offering valuable insights into the differential responses of tobacco cultivars to heavy metal exposure.Furthermore, Abuzaid et al (2022) conducted a study examining the long-term effects of irrigation with sewage effluents and contaminated groundwater on metal accumulations in sandy soils and the edible parts of food crops [70].This study underscored the notable spatial variations in total and available metal contents in soils, a consequence of continuous irrigation.It underscored the necessity for adequate water treatment and appropriate remediation techniques to mitigate metal accumulation in crops.Additionally, Hattab-Hambli et al (2016) managed coppercontaminated soils through aided phytoextraction in field plots, implementing a crop rotation of tobacco and sunflower [71].Their investigation into the mobility and phytoavailability of Cu, Zn, Cr, and Pb over two and three years revealed a decrease in total concentrations in the soil pore water, likely attributable to annual plant shoot removals and leaching.Lastly, a study by Száková et al (2018) explored the capacity of German chamomile to accumulate risk elements (As, Cd, Pb, Zn) in various soils [21].The findings indicated that while As, Pb, and Zn predominantly remained in the roots, Cd demonstrated a notable ability to translocate to the shoots, with the bioaccumulation factor for Cd being the highest among the elements studied.This research suggests that chamomile could serve as an effective alternative crop for soils contaminated with risk elements.
While the primary concern in the research locale pertains to Pb, followed by Zn, our study has elucidated models based on soil determinants related to heavy metals in tobacco leaves.These models not only offer precise predictions regarding the status of Cr, Pb, and Zn in the region's tobacco leaves but also provide insights essential for the safe cultivation of tobacco in the studied area.

Conclusion
Within the designated research area, the average concentrations of Cr, Pb, and Zn in the soil were determined to be below the secondary thresholds established by both China and Fujian Province.Notably, only 2.93% and 0.472% of the samples exceeded these standards for Pb and Zn, respectively.The absence of any soil samples surpassing the Cr threshold suggests an absence of Cr contamination in this region.The enrichment coefficients for Pb and Zn were calculated at 23.1 and 19.7, respectively, while Cr exhibited a significantly lower enrichment coefficient of 5.67.
Through rigorous correlation and stepwise regression analyses, key soil parameters influencing the concentrations of Cr, Pb, and Zn in tobacco leaves were discerned.These parameters include soil pH, essential nutrients such as K and N, OM content, and the inherent concentrations of Cr, Pb, and Zn in the soil.Specifically, soil pH demonstrated an inverse relationship with Pb content in tobacco, while organic matter showed a negative correlation with Cr content.Available phosphorus was inversely related to Zn content in tobacco.In contrast, Alkali-hydrolyzable Nitrogen and Available Potassium exhibited a direct correlation with the Cr content in tobacco, and organic matter was positively associated with its Zn content.
The predominant concern regarding heavy metal contamination in the study area is primarily associated with Pb, followed by Zn.Implementing soil remediation techniques or cultivating tobacco varieties with inherent resistance to Pb and Zn emerges as a pragmatic approach to address and manage heavy metal contamination.The predictive models developed in this study for heavy metals in tobacco leaves have achieved a significant level of accuracy, providing a robust scientific basis for utilizing soil property indices in the study area to anticipate the potential risk of heavy metal contamination in tobacco leaves.
It is imperative to underscore that, beyond soil characteristics, various factors, including size distribution of the soil particles, and mineralogy, plant species, root structure, growth stage, ambient temperature, humidity, and other environmental conditions, play a certain role in the accumulation of Cr, Pb, and Zn in plants.While this study primarily focused on the salient factors influencing soil physicochemical properties, there exists an opportunity to refine and enhance the model's predictive accuracy by incorporating these additional variables.

Figure 1 .
Figure 1.Location map of the study area (Longyan City).

Figure 2 .
Figure 2. Scatter plot illustrating the predicted versus actual measurements of Cr, Pb, and Zn concentrations.

Table 1 .
An overview of the physical and chemical characteristics of soils in areas cultivated with tobacco of Longyan.

Table 2 .
Summary statistics for total and bioavailable concentrations of Cr, Pb, and Zn in soil and tobacco leaf samples.Indicates no data available or not applicable, due to the absence of established standards for heavy metal content in tobacco leaves.

Table 3 .
Correlational analysis between heavy metal concentrations and other variables in tobacco and soil samples.

Table 4 .
Predictive models for concentrations of Cr, Pb, and Zn in tobacco leaves.Organic Matter; X Cr , X Pb and X Zn represent the metal content of Cr, Pb and Zn in soil; In the prediction model of Cr, Pb and Zn, X Cr, X Zn , and X Pb denote the metal concentration of Cr, Pb and Zn in soil respectively.